1. Field
The present disclosure generally relates to antennas and, in particular, to vertical array antennas having a ground plane.
2. Description of the Related Art
Maintaining a communication link between a satellite in low-Earth orbit (LEO) and a communication station on the ground is a challenge as the satellite flies by at roughly 17,000 miles per hour. The satellite's communication antenna must be able to either steer the communication beam or have a very broad beam pattern. Some satellites, such as the Hubble Space Telescope, incorporate a physically steerable parabolic dish that can track the ground station. Another type of steerable antenna, referred to as a “phased array”, uses multiple fixed antennas and adjusts the timing of the signals from each antenna to steer the beam. Some satellites frequently use a single fixed antenna designed to provide a broad beam. As the majority of the Earth's surface visible from a LEO satellite is at a significant angle to the nadir axis, it is desirable for the satellite antenna to have good coverage at angles of 30-80 degrees from the nadir axis. A null spot in the beam pattern directly on the nadir axis is sometimes acceptable as the amount of ground coverage lost is a very small part of the total contact area.
Antenna size and complexity are also important factors in satellite design. Particularly for small satellites, there may be height or width limits on the size of the antenna to avoid interference with adjacent antennas or to fit inside the launch vehicle fairing. The number of elements of an antenna also drives cost and weight both directly and indirectly, as larger numbers of elements require larger and stronger supporting structure. Minimizing the volume and complexity of an antenna, and thus the number and the size of the elements, is desirable.
Designing an antenna with multiple elements enables the designer to shape the beam and achieve a higher gain than possible with a single radiative element. One design approach is to stack multiple antenna elements in a vertical array. The spacing between elements is limited on the low end by mutual coupling effects, and is limited on the high end by the creation of interfering secondary lobes as the spacing approaches one wavelength. A “rule of thumb” that balances these factors is to use a half-wavelength for the inter-element spacing in vertical arrays.